In electrophotography, an electrostatic latent image formed on a photoreceptor provided with a photoconductive material is developed with colorant-containing toner particles, and then, the resulting toner image is fixed onto a transfer material such as paper by heat, pressure or the like, thereby forming a copy or printed matter.
In order to improve physical properties of the toner, it is generally carried out to add an external additive (surface treating agent) to the toner particles. However, the toner particles have a size distribution, so that even when the external additive is added to the toner particles, there is a distribution in charging amount. Accordingly, even a negatively chargeable toner inevitably contains positively charged toner particles. In an image forming apparatus preparing an image by negative charge reversal development, there is the problem that the toner adheres to a non-image area of an image carrier (photoreceptor), the amount of the toner adhered to cause fogging further increases with an increase in the number of printed sheets, correlatively with deterioration of the toner, and the load of cleaning in the photoreceptor also increases. In particular, when the toner is used as a full color toner, a selective development occurs by a broadened charge distribution of the toner derived from the size distribution of the toner particles, or when positively charged toner particles exist, not only the problem of fogging but also the problem of the development of a “reverse transfer toner” reversely transferred to the photoreceptor in color superposition is encountered.
In a single-component non-magnetic toner, when a large amount of silica particles are added in order to prevent deterioration of the toner, the fluidity of the toner is secured, but the fixability is reduced. Further, in order to enhance the negatively chargeable capacity of the toner, it is known to add positively chargeable titania having a polarity opposite to that of the toner together with silica particles. However, positively chargeable titania is liberated from surfaces of the toner particles in some cases, with an increase in the number of printed sheets, which poses the problem with respect to the maintenance of image density in continuous printing. Further, when the negatively chargeable capacity of the toner becomes too high, the printed image density is reduced. To prevent this, it is also known to use titania having a relatively low electric resistance and a large primary particle size for preventing the titania particles from being embedded. However, an increase in the number of printed sheets raises the problem that titania is librated from surfaces of toner mother particles, resulting in failure to sufficiently exhibit the effects. Further, in order to prevent filming, it is also known to add aluminum oxide (alumina) particles together with silica particles. However, it is difficult to subject to hydrophobic treatment the alumina particles, and the introduction of excessive alumina particles raises the problem of shortening the life of a photoreceptor.
Further, when various metal oxide particles are externally added, it is known to adding particles having a relatively large particle size and particles having a relatively small particle size in combination, thereby improving the fluidity, cleaning property, transfer characteristics, charge property and “durability” (prolonging the life of the toner) of the toner. Furthermore, it is also known to specify the liberating rate of an external additive from toner mother particles, thereby making possible the charge stabilization, antifogging, white spot prevention, maintenance of image density and filming prevention. In addition, it is known that combined oxide particles comprising aluminum oxide and silicon dioxide are used as an external additive, thereby being able to obtain toner particles excellent in fluidity and showing stable charge behavior (reference 1). However, in any case, when the toner is applied to a full color toner, a selective development occurs by a broadened charge distribution of the toner derived from the size distribution of the toner particles, or the existence of positively charged toner particles poses not only the problem of fogging but also the problem of the development of a “reverse transfer toner” reversely transferred to the photoreceptor in color superposition.
Further, it is known to modify surfaces of single fine particles or parts thereof with another material, instead of using plural kinds of external additives in combination in toner particles, to obtain the effects of the plural kinds of external additives, such as the fluidity, charge property, maintenance of image density and fogging or filming prevention. For example, a toner for electrophotography is known in which hydrophobic fine particles obtained by coating fine silica particles with a hydroxide or an oxide of one or more of titanium, tin, zirconium and aluminum in an aqueous system, and further coating surfaces thereof with an alkoxysilane are used as an external additive (reference 2). Further, a toner is proposed in which silica-containing titanium oxide particles obtained by coating outer surfaces of silica particles with titanium oxide are used as an external additive (reference 3). According to the addition of such particles, it is possible to decrease the amount of positively charged toner particles, but the average charging amount is insufficient. Accordingly, neither of them achieves both objects of improving the transfer efficiency and reducing the reverse transfer toner.
[Reference 1]                JP-A-2000-181130 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)        
[Reference 2]                JP-A-2002-29730        
[Reference 3]                JP-A-2002-148848        
[Reference 4]                Japanese Patent No. 2533067        
[Reference 5]                JP-A-2001-300083        
The present invention relates to a negatively chargeable toner having a novel combination of external additives, which provides little fogging in a non-image area caused by a toner on a photoreceptor in development and can prevent excessive charge, resulting in improvement in the transfer efficiency and the prevention of the development of a reverse transfer toner on the photoreceptor in superposition of the second color or later, a method for producing the same, and a full color image forming apparatus using the negatively chargeable toner.